Fourteen people have been arrested in Belgium after authorities foiled a plot to free an al-Qaeda member arrested in September 2001, officials say.

The suspects, detained in police raids, were described as Islamic militants.

They were seeking to free Tunisian Nizar Trabelsi, jailed in Belgium for planning to attack US targets.

An official said the plot involved explosives and that security was being stepped up in the capital over fears of a "possible attack".

Belgium hosts the main European Union institutions and Nato headquarters, as well as a raft of multi-national companies.

NIZAR TRABELSI

Tunisian, 37, played football for several German teams

Arrested in September 2001 in Belgium for plotting attacks on US targets

Sentenced to maximum 10 years in prison in 2003

Planned to attack Kleine Brogel air base where US military personnel stationed

Testified he intended to kill American soldiers

The arrests were announced by federal prosecutors and officials from the country's crisis co-ordination centre, at a hastily arranged news conference.

"They were planning to use weapons and explosives to free him... These means could be employed for another use," officials said.

Security was being stepped up at public sites including airports, subway stations and Christmas markets although the authorities said they were not aware of a specific threat.

Trabelsi, who used to play professional football in Germany, was jailed for 10 years in 2003 for planning to blow up a military base in Belgium housing US soldiers.

The court heard that Trabelsi had met al-Qaeda leader Osama Bin Laden several times in Afghanistan and asked to become a suicide bomber.

He said he was ordered to go to Belgium, pack a bomb into a lorry and blow it up - with himself at the wheel - next to the canteen of the Kleine Brogel military base about 100 miles (160km) from Brussels.

But Trabelsi was arrested in Brussels two days after the 11 September 2001 attacks on the United States.

Defence lawyers said the court had ignored the remorse Trabelsi had shown during the trial and had made an example of him after the US terror attacks.